Plastic containers are used for packaging and containing purposes in various fields such as beverages, foods, toiletry goods, and medicines because they not only have a superior hardness, light weight, and superior moldability, but are also cost efficient, difficult to break, and easy to re-seal.
Although plastic containers have these advantages, plastic containers also have disadvantages in that a low molecular gas such as oxygen and carbon dioxide can be transmitted therethrough. Such a disadvantage is called low gas impermeability because gas impermeability is low. Occasionally, some of the contents in the container are affected undesirably by such gases. Here, various attempts for improving the gas impermeability of plastics have been made when producing containers industrially. Among such attempts, one method is realized, in which a material having a high gas impermeability and made of an inexpensive common material is formed in a multi-layer structure.
However, it is difficult to recycle a material having a multi-layered structure which consists of at least two kinds of material. Thus, there was a problem from an ecological point of view because a material having a multi-layer structure must be discarded after usage. Therefore, other attempts have been made for reducing a material having a high gas impermeability as much as possible until the material having a high gas impermeability has no effect on recycling. However, quite often, it was not possible to realize a desirable gas impermeability in a material having a multi-layer structure.
Currently, in order to realize a recycling ability and gas impermeability to oxygen, carbon monoxide, and steam, a method has been proposed in which a thin film having gas impermeability is formed on the inner surface of a container made by commonly-used plastics. One thin film forming method is a plasma-assisted CVD method, in which a thin film is formed on the inner surface of a container by plasmatizing a process gas and reacting the process gas chemically. Specifically, the plasma-assisted CVD method, in which a container is disposed between a hollow high frequency electrode having a shape which is approximately the same as the outer shape of the container, and an inner electrode having a shape which is approximately the same as the inner shape of the container, is known in the art (see, for example, patent document 1). As another form of the plasma-assisted CVD method, both the high frequency electrode and the inner electrode are disposed away from a surface of the container by approximately the same distance (see a patent document 2).
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H8-53117
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H8-175528
However, even if a thin film is formed by these methods, it is still difficult to control strictly flow ratio of a reactive gas and a monomer gas in the plasmatized process gas. As a result, there has been a problem in that it was not possible to stably form a thin film having a sufficient gas impermeability, and in that, the gas impermeability varied even among the produced containers. Furthermore, there has been another problem in that the gas impermeability of the thin film decreases because the produced thin film does not have sufficient flexibility, and finally, cracking occurs on the thin film when using the container.
Also, a thin film has been formed on the inner surface of not only plastic container, but also, for example, glass container in order to prevent lead, cadmium, etc., from melting in the contents of the containers. For that case, it is necessary to form the thin film stably without variation.
Also, a thin film having gas impermeability has been formed on the inner surface of plastic containers in order to add gas impermeability. For that purpose, a plasma-assisted CVD method (hereinafter, a plasma CVD method), in which a thin film is formed on the inner surface of the plastic container by plasmatizing a process gas and reacting the plasmatized process gas chemically, is known in the art.
It was not possible to know whether or not the thin film has a desirable surface quality during forming the thin film by the plasma CVD method. Therefore, conventionally, a thin film has been produced while monitoring parameters (for example, degree of vacuum, applied power, and introduced gas flow amount), and after that, whether or not the produced thin film had a desirable surface quality has been evaluated. However, the monitoring of the parameters, such as the degree of vacuum, applied power, and introduced gas flow amount, has not yet achieved the desirable surface quality of the produced thin film. Therefore, a more improved process monitoring method is needed.
For that purpose, a method in which a plasma emission is monitored has been proposed. This method is called a plasma diagnosis, in which it is possible to obtain information concerning the actual inner structure of a plasma by monitoring emission of the plasma. By employing this method, it is possible to forecast the surface quality of the thin film accurately.
For example, a method is proposed in which whether or not a process is conducted properly is determined by monitoring a ratio between the intensity of hydrogen alpha rays and the intensity of hydrogen beta rays, which are radiated from a plasma, or by monitoring a ratio between the hydrogen alpha rays or beta rays, and the intensity of helium radiation, in Japanese Unexamined Patent Application, First Publication No. H1-87777 (patent document 3).
However, the method which is disclosed in Japanese Unexamined Patent Application, First Publication No. H1-87777, has the following problems.
Firstly, although the hydrogen alpha rays have a relatively large intensity, the hydrogen beta rays have a small intensity, and the hydrogen beta ray vary greatly. Therefore, when the ratio between the intensity of the hydrogen beta rays and the intensity of the radiation of other atomic specimen (molecular specimen) is calculated, the calculated ratio varies greatly; thus, it is difficult to know the inner structure of the plasma accurately.
Also, even if the ratio of the intensities is calculated between relatively intense rays such as the hydrogen alpha rays and the helium radiation, both the intensity of the hydrogen alpha rays and the intensity of the helium radiation fluctuate similarly if the thin film forming pressure fluctuates. Therefore, even if the thin film forming pressure fluctuates and the surface quality of the produced thin film fluctuates accordingly, the ratio between the intensity of the hydrogen alpha rays and the intensity of the helium radiation does not vary greatly. Therefore, there is a problem in that it is not possible to monitor the plasma accurately.
Also, the spectrum of the plasma is measured in a wide range from a visible wavelength to a near-visible wavelength (see FIG. 3 in patent document 3). There is a problem because a specific complicated spectrometer is necessary for such a measurement, and such a spectrometer is expensive.
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. H1-87777.